Signal quadrature is used to generate signals (known as quadrature signals) with four different phases (for example, 0°, 90°, 180°, and 270°) from a single or differential input into a filter, where the input signal has a set frequency and set amplitude. Signal quadrature is an important function for modern radio modulators and demodulators and is used in modern transceiver signal processing systems. When using signal quadrature, the accuracy of the phase angles as well as the bandwidth accuracy of the quadrature signals generated from the input signal is critical and directly correlates with the ability to accurately send and detect quadrature information laden signals.
One way to generate quadrature signals is by using a polyphase filter, and more specifically, a quadrature generator. The quadrature generator consists of a simple ring of capacitor and resistor pairs in which four symmetrical points in the ring are tapped in order to obtain quadrature signals having four phases (for example, 0°, 90°, 180°, and 270°) of an input signal. The resistor and capacitor values are chosen such that the corner frequencies of the quadrature generator coincide with the frequency of the input signal used to generate the quadrature signals from. The response of the quadrature generator is inherently narrowband with quadrature phase accuracy and amplitude mismatch degrading as the input frequency departs from the capacitor and resistor pairs' corner frequencies. Cascading several rings with staggered corners tends to broaden the bandwidth capability of a device, but generally this may result in higher costs, more area used, a larger current drain, higher noise and more signal loss. Even with cascading multiple rings, it is difficult to maintain quadrature accuracy over one octave of bandwidth.
In typical direct conversion and direct launch transceiver systems, obtaining good bandwidth match between a pair of channels can also be challenging. Tracking circuits do well to shift some of the absolute bandwidth errors, but are unable to correct out differences between two matched filter responses.
As a result, it would be desirable to provide a method of tuning a filter or a pair of filters to correct for any mismatches in phase angle, amplitude, and corner frequency in a signal, such as a quadrature signal. Additionally, it would be desirable to provide a method of tuning a pair of filters so that the signals generated by the pair of filters have accurate phase angles.